The positron beam technique is shown to be a sensitive indicator of the presence of arsenic within the grain boundaries of poly-silicon. Variable-energy positron beam and secondary-ion mass spectrometry studies have been performed on As+-implanted pre-amorphized Si samples as a function of dose and rapid thermal anneal temperature. Positron trapping within negatively-charged grain boundaries of the recrystallized poly-Si is observed, resulting in a similar to 2% elevation in the Doppler-broadening S lineshape parameter value. Infusion of As+-ions into the grain boundaries passivates the charge and reduces their specific positron-trapping rate.